This invention concerns the mixing of rubber. In particular, it relates to the continuous preparation of rubber compounds containing silica as reinforcing agent.
U.S. Pat. No. 5,626,420 discloses a continuous mixing apparatus for preparing a rubber compound. This apparatus has an elongated mixing chamber containing at least one rotor in rotation in a stator, an upstream end and a downstream end, between which the material progressively flows, one or more inlets through which said mixing chamber receives the constituents of the rubber compound and an outlet at the downstream end through which the rubber compound is discharged. The continuous mixing apparatus has the advantage, notably, of making it possible to obtain rubber compounds in which the reinforcing agent--e.g., carbon black--is remarkably well dispersed.
Patent EP 0,501,227 showed that rubber compounds containing silica as reinforcing agent in very large proportions could have remarkable properties for use as a tire tread, on condition, notably, that the silica should have an excellent dispersion in the elastomeric matrix.
It was therefore sought to use a continuous mixer, such as presented in U.S. Pat. No. 5,626,420, in order to employ rubber compounds containing silica as reinforcing agent.
It has been discovered, contrary to expectations, that the use of such a mixer did not make it possible, in all cases, to obtain satisfactory rubber compounds, notably, when those compounds contain silica as reinforcing agent. In particular, instabilities of flow of material into the mixer were sometimes observed. Furthermore, the properties of those compounds proved to be different from those obtained with the known "batch" mixing methods: For example, greater hardness and reduced calcining time (time necessary, at a given temperature, for the vulcanization reaction to start in a sample of the rubber compounds after mixing).